1. Field of the Invention
Exemplary aspects of the present invention relate to a fixing device and an image forming apparatus, and more particularly, to a fixing device for fixing a toner image on a recording medium and an image forming apparatus including the fixing device.
2. Description of the Related Art
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile functions, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of an image carrier; an optical writer emits a light beam onto the charged surface of the image carrier to form an electrostatic latent image on the image carrier according to the image data; a development device supplies toner to the electrostatic latent image formed on the image carrier to make the electrostatic latent image visible as a toner image; the toner image is directly transferred from the image carrier onto a recording medium or is indirectly transferred from the image carrier onto a recording medium via an intermediate transfer member; a cleaner then cleans the surface of the image carrier after the toner image is transferred from the image carrier onto the recording medium; finally, a fixing device applies heat and pressure to the recording medium bearing the toner image to fix the toner image on the recording medium, thus forming the image on the recording medium.
Such fixing device may include a fixing belt or a fixing film to apply heat to the recording medium bearing the toner image. FIG. 1 is a sectional view of a fixing device 20R1 including a fixing belt 204. The fixing belt 204 is looped around a heating roller 202 and a fixing roller 203. A pressing roller 205 presses against the fixing roller 203 via the fixing belt 204 to form a fixing nip N between the pressing roller 205 and the fixing belt 204. The fixing belt 204 is heated by a heater 201 provided inside the heating roller 202. As a recording medium P bearing a toner image passes between the fixing belt 204 and the pressing roller 205, the fixing belt 204 and the pressing roller 205 apply heat and pressure to the recording medium P bearing the toner image to fix the toner image on the recording medium P.
However, the heating roller 202 has a relatively large heat capacity, resulting in a longer warm-up time of the fixing device 20R1.
To address this problem, the fixing device may include a fixing film having a relatively small heat capacity. FIG. 2 is a sectional view of a fixing device 20R2 including a fixing film 213. A pressing roller 212 presses against a ceramic heater 211 provided inside the fixing film 213 via the fixing film 213 to foam a fixing nip N between the pressing roller 212 and the fixing film 213. As a recording medium bearing a toner image passes between the pressing roller 212 and the fixing film 213, the fixing film 213 heated by the ceramic heater 211 and the pressing roller 212 apply heat and pressure to the recording medium bearing the toner image to fix the toner image on the recording medium.
However, the fixing film 213 has a drawback in that sliding over the ceramic heater 211 increases friction between the fixing film 213 and the ceramic heater 211 over time, resulting in unstable movement of the fixing film 213 and increased driving torque of the fixing device 20R2. Further, the rotating fixing film 213 is heated by the ceramic heater 211 at the fixing nip N only, and therefore the fixing film 213 is at its lowest temperature just before the fixing nip N in a direction of rotation of the fixing film 213, generating faulty fixing due to low temperature at the fixing nip N when the fixing film 213 is rotated at high speed.
To address the above-described problems, the fixing device may include a resistance heat generator provided inside a fixing belt, with a slight gap between the resistance heat generator and the fixing belt. However, when the fixing belt rotates, slack appears in the fixing belt downstream from the fixing nip in a direction of rotation of the fixing belt, widening the gap between the resistance heat generator and the fixing belt. The wider gap may delay heat transmission from the resistance heat generator to the fixing belt, and therefore the temperature of the resistance heat generator may increase abnormally at the downstream position.